A retrovirus isolated originally from a California wild mouse, induces chronic myelogenous leukemia. This unique virus provided an opportunity to follow the development of this disease during the preleukemic (11 mice) and leukemic phases (14 mice). Stem cells, including CFU-E, BRU-E, CFU-S, and CFU-GM were enumerated and the stage of disease evaluated by differential counts of marrow and spleen cills, spleen weight nd histology. During the preleukemic phase no severe alterations occurred for any aspect measured. Six months after inoculation the number of CFU-GM increased modderately in the spleen. CFU-S could not be measured in the leukemic mice but were moderately elevated in preleukemics. The result of this study, indicating a lack of a distinguishable preleukemic phase, is being reevaluated in an ongoing second study.